As an example of an infrared image pickup device, there is known a bolometer infrared image pickup device including a linear or area sensor array and a readout circuit (Patent Literature 1). An example thereof is described below with reference to FIG. 4.
FIG. 4 partially illustrates a readout circuit and an area sensor array in the bolometer infrared image pickup device.
This infrared image pickup device includes thermoelectric conversion elements arranged in a two-dimensional matrix, detects an infrared signal received by the thermoelectric conversion element for each thermoelectric conversion element, and outputs a detection signal as an electric signal. This infrared image pickup device can perform parallel processing of the detection signals by pixel switches 201 selected by a vertical shift register 205 via a scanning line 211, and a readout circuit 206 connected to thermoelectric conversion elements 202 via a horizontal switch 204 and a signal line 203. Outputs of a plurality of readout circuits 206 are sequentially output to the outside from an output terminal 210 by a horizontal shift register 208. Reference numerals 207 and 209 denote a multiplexer switch and an output buffer, respectively.
A structure of the readout circuit of FIG. 4 is illustrated in FIG. 5. A readout circuit 101 includes a bias circuit 102 for applying a constant voltage to a bolometer element (thermoelectric conversion element) 105, a bias-canceling circuit 103 for removing offset currents of components other than a signal of a subject, and an integration circuit 104 including an operational amplifier (hereinafter referred to as an integration operational amplifier) 111 connected to both the bias circuit 102 and the bias-canceling circuit 103. The plurality of readout circuits 101 are supplied with an input voltage via input voltage wirings 1 and 2 (106 and 107) so as to perform reading operation simultaneously in parallel.
The operation is generally as follows. A variation of resistance of each bolometer element 105 is generated in accordance with intensity of infrared incident light from the subject and is detected as a difference between a bolometer current and a bias cancel current determined by input voltages VB1 and VB2 (115 and 121). The detected current difference is integrated by the integration circuit 104 and simultaneously undergoes current-to-voltage conversion to be output as a voltage value.
Specific operations of the bias circuit 102 and the bias-canceling circuit 103 are as follows. First, the input voltages VB1 and VB2 are adjusted in a state where a shutter of the image pickup device is closed (without incident light from the subject). Thus, a current flowing in the bolometer element 105 is balanced with a current flowing in a bias-canceling resistor 116. After that, the shutter is opened so as to extract only a current that has been varied due to resistance variation of the bolometer element 105 caused by the incident light from the subject. Details of individual circuits of FIG. 5 are described below.
First, the bias circuit 102 includes an NMOS transistor (hereinafter referred to as a bias transistor) 108 having a source connected to one terminal of the bolometer element 105, and a source follower circuit 109 having an input terminal connected to the input voltage wiring 1 (106) and an output terminal connected to a gate of the bias transistor 108. Because the source follower circuit 109 drives the bias transistor 108 with low impedance, intrusive noise in each readout circuit can be suppressed. The bias circuit 102 applies a constant voltage to each bolometer element 105. Thus, the resistance variation of the bolometer element 105 is converted into a current value.
Further, a VGS removing voltage generation circuit 1 (110) is a circuit that compensates a gate-source voltage (VGS) of each of the bias transistor 108 and a transistor in the source follower circuit 109 and has a circuit structure in which a voltage variation of VGS does not appear in a drain current (compensation of voltage variation). More specifically, the VGS removing voltage generation circuit 1 (110) includes the bias transistor 108 and the source follower circuit 109 that are the same as those of the bias circuit 102, and an operational amplifier 114. Connections are made as follows. A source of the bias transistor 108 is connected to one terminal of the bolometer element 105. A gate of the bias transistor 108 is connected to an output terminal of the source follower circuit 109, and a drain of the bias transistor 108 is connected to +5 V. The operational amplifier 114 has an output terminal connected to an input terminal of the source follower circuit 109, an inverting input terminal (−) connected to the source of the bias transistor 108, and a non-inverting input terminal (+) connected to the input voltage VB1 (115).
Similarly, the bias-canceling circuit 103 includes the resistor element (bias-canceling resistor) 116 having one terminal connected to a power source, a PMOS transistor (hereinafter referred to as a canceler transistor) 117 having a source connected to the other terminal of the resistor element, and a source follower circuit 118 having an input terminal connected to the input voltage wiring 2 (107) and an output terminal connected to a gate of the canceler transistor 117.
Here, an infrared signal has a large offset component, and a signal component from the subject with a very small level exists on the offset component. Therefore, this bias-canceling circuit 103 is constituted for a purpose of removing the offset component. In addition, because the source follower circuit 118 drives the canceler transistor 117 with low impedance, intrusive noise in each readout circuit 101 can be suppressed.
In addition, similarly to the VGS removing voltage generation circuit 1 (110), a VGS removing voltage generation circuit 2 (119) is also a circuit for compensating VGS of each of the canceler transistor 117 and a transistor in the source follower circuit 118. More specifically, the VGS removing voltage generation circuit 2 (119) also includes the bias-canceling resistor 116, the canceler transistor 117, the source follower circuit 118 that are the same as those of the bias-canceling circuit 103, and an operational amplifier 120. Connections are made as follows. A source of the canceler transistor 117 is connected to one terminal of the bias-canceling resistor 116. A gate of the canceler transistor 117 is connected to an output terminal of the source follower circuit 118. A drain of the canceler transistor 117 is connected to +5 V. The operational amplifier 120 has an output terminal connected to an input terminal of the source follower circuit 118, an inverting input terminal (−) connected to the source of the canceler transistor 117, and a non-inverting input terminal (+) connected to the input voltage terminal VB2 (121).
Drains of the bias transistor 108 and the canceler transistor 117 in the readout circuit 101 are connected to the inverting input terminal (−) of the integration operational amplifier 111 and one terminal of an integration capacitor 122, which are provided in the integration circuit 104. The integration circuit 104 integrates current variation of the bolometer element 105 described above.
The other terminal of the integration capacitor 122 is connected to an output terminal of the integration operational amplifier 111, and a non-inverting input terminal (+) of the integration operational amplifier 111 is connected to +5 V. Thus, the inverting input terminal (−) of the integration operational amplifier 111, namely the drains of the bias transistor 108 and the canceler transistor 117 are usually fixed to +5 V. A voltage of the integration capacitor 122 after the integration is extracted from the output terminal of the integration operational amplifier 111 and is sequentially output as an output signal from each readout circuit 101. In addition, a resetting switch 123 is disposed between the inverting input terminal (−) and the output terminal of the integration operational amplifier 111. After outputting the integrated voltage from the integration capacitor 122, the switch 123 is turned on so that the drains of the bias transistor 108 and the canceler transistor 117 are set to +5 V that is the voltage of the non-inverting input terminal (+) of the integration operational amplifier 111.
A clipping diode A (124) and a clipping diode B (125) are connected to the inverting input terminal (−) of the integration operational amplifier 111. When one of a current flowing in the bolometer element 105 and a current flowing in the bias-canceling circuit 103 becomes excess so that an output of the integration operational amplifier 111 is saturated, these diodes function to compensate the excess current.
Next, a mechanism of high temperature aliasing and temperature drift is described with reference to FIGS. 6A and 6B.
In FIG. 6A, the input voltage wirings connected to the plurality of readout circuits and the bias transistor or the canceler transistor are directly connected, and hence there is a problem in that the input voltage is effected by a voltage change inside the readout circuit when light enters from a high temperature subject. The drain voltage of each transistor described above is usually fixed to +5 V. However, when light enters the bolometer element (thermoelectric conversion element) from a high temperature subject, the voltage may change. When light enters from a high temperature subject, a resistance of the bolometer element is decreased. Therefore, in the case where the current supplied to the bolometer element increases, the current flows rapidly from a capacitor in the integration operational amplifier to the bolometer element through a path indicated by an arrow in the circuit on the right side of FIG. 6A at an instant when a resetting switch RSTSW (the switch 123 of FIG. 5) is turned off in starting the integration. As a result, an imaginary short of the operational amplifier is lost, and a voltage variation occurs at the inverting input terminal (−) of the integration operational amplifier (period t1 of FIG. 6B).
After that, also when the output of the integration operational amplifier is saturated, the imaginary short of the operational amplifier is lost, and the voltage at the inverting input terminal (−) of the integration operational amplifier is ultimately decreased from +5 V to approximately +4.3 V as a clipping voltage (period t1′ of FIG. 6B). In such a voltage variation of the inverting input terminal (−), the drain voltages of the bias transistor and the canceler transistor are decreased. Therefore, the gate voltages of the transistors also vary due to parasitic capacitance between the drain and the gate thereof, and hence a voltage variation occurs in each input voltage wiring connected to each gate.
Therefore, when light from a high temperature subject enters some bolometer elements, because of the influence of the voltage variation (intrusive noise) described above, a displacement current is generated in every readout circuit via the input voltage wiring 1 and the input voltage wiring 2. The generated displacement current affects output voltages of other bolometer elements which light does not enter from a high temperature subject.
As a countermeasure thereagainst, the image pickup device of Patent Literature 1 uses source follower circuits (109 and 118 of FIG. 5) and drives a bias transistor and a canceler transistor at low impedance, so as to suppress a variation of the gate voltage due to high temperature aliasing.